1. Field of the Invention
The present invention relates to a multi-band U-slot antenna, more particularly, which has a plurality of frequency notch filters to operate in a multi-band frequency as a single antenna.
2. Description of the Related Art
With a variety of systems incorporated together lately, an antenna has been required to possess higher capabilities. The antenna needs to function not only in a single narrow frequency band but also in a multiple narrow frequency band or a broad frequency band. Thus, the antenna should be designed to achieve multi-band performance. The multi-band can be attained by two methods of combining narrow bands together or embedding a frequency notch filter in a broad band. The former entails a complicated design and a time-consuming tuning caused by undesired results from design or during manufacturing. Therefore, recently, the latter has gained a spotlight. That is, a slot functioning as a notch filter has been inserted into a broadband antenna. This method involves simple designing and does not require a separate tuning after manufacturing, thereby simplifying a process.
FIG. 1 is a perspective view illustrating a conventional planar antenna with a single U-slot.
As shown, a single U-slot microstrip patch antenna has a U-slot 13 formed in a patch 12. Although not illustrated, a dielectric body 12 and a ground substrate are stacked sequentially under the patch 12. Also, a coaxial line extends through the ground substrate and the dielectric body 11 onto the patch 12.
The U-slot located adjacent to radiating edges of the patch 12 disturbs distribution of current which generates fundamental resonance mode, thereby generating another resonance in its near-by frequency. This resonance characteristic, in combination with resonance of a square microstrip patch, beneficially assures dual resonance characteristics. That is, primary resonance is generated by the microstrip patch 12 and secondary resonance is generated by the U-slot 13. In FIG. 1, □+□+□ indicate a length of a current path where the primary resonance occurs and □+□+□+□+□ indicate a length of the current path where the secondary resonance occurs.
The two resonance frequencies, when spaced apart from each other, realize a dual resonance antenna, i.e., with a single notch filter. Meanwhile, the resonance frequencies, when located substantially identically, provide a broadband antenna. In general, the dual resonance antenna exhibits a big loop and a small loop on an impedance trajectory of a smith chart. Notably, position and size of the small loop within the big loop determines a bandwidth of impedance of the antenna. Parameters for varying the small loop on the smith chart include width and length of a square patch (bottom of the U-slot), length and shape of the U-slot, thickness and relative permittivity of the substrate.
Conventionally, attention was drawn only to a single notch filter, which was thus embedded in an antenna to achieve multi-band performance. However, little consideration was given to a method for embedding the notch filter to implement dual or more bands. The conventional single notch filter, when adopted for such multi band performance, is accompanied by great problems.